1. Field of the Invention
The present invention relates to an engine oil degradation-determining system and method, and an engine control unit, for determining a degradation level of engine oil for lubricating an internal combustion engine.
2. Description of the Related Art
Conventionally, an engine oil degradation-determining system of the above-mentioned kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 62-203915 (this system will be hereinafter referred to as “the first degradation-determining system”). The first degradation-determining system, which determines degradation of engine oil used in an engine for a vehicle, includes an oil temperature sensor for detecting the temperature of the engine oil, an engine rotational speed sensor for detecting the rotational speed of the engine, a control unit for calculating the service life of the engine oil, and a display for displaying the calculated service life of the engine oil.
In the first degradation-determining system, a degradation coefficient of the engine oil is set e.g. according to the detected oil temperature and engine rotational speed, and the travel distance of the vehicle, and then a numerical value indicative of the effective engine oil usage is calculated based on the degradation coefficient. Further, the calculated numerical value is subtracted from a value indicative of the effective service life of the engine oil (hereinafter referred to as “the effective service life value”) to thereby obtain a numerical value indicative of the remaining life of the engine oil (hereinafter referred to as “the remaining life value”). The calculated remaining life value is displayed on the display, as a proportion to the effective service life value, so as to notify the driver. Further, when the remaining life value becomes smaller than a predetermined value, a warning to the effect that an oil change is needed is displayed on the display. Then, when a manual reset switch is operated after execution of the oil change, the travel distance of the vehicle, the engine rotational speed, and so forth are reset to respective predetermined values.
Another conventional engine oil degradation-determining system has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 62-55407 (this system will be hereinafter referred to as “the second degradation-determining system”). The second degradation-determining system, which also determines degradation of engine oil used in an engine for a vehicle, includes a level sensor switched on and off depending on whether or not the oil level of the engine oil becomes lower than a predetermined limit level, an engine hood switch for detecting opening and closing of an engine hood for replenishment of the engine oil, and an arithmetic operation circuit for calculating a time to change the engine oil. In the second degradation-determining system, when it is detected that the level sensor has been switched on in a state in which an ignition switch has been turned off and the engine hood is open, it is judged that a predetermined sufficient amount of engine oil has been replenished, and the time to change the engine oil is prolonged.
In the above first degradation-determining system, however, the engine rotational speed and the travel distance of the vehicle as parameters used in calculating the effective engine oil usage are reset only when the manual reset switch is operated after an oil change. For this reason, when a driver forgets to operate the reset switch, the fact that the remaining life of the engine oil is extended by the oil change is not reflected in the calculation of the remaining life, and hence the remaining life value is erroneously calculated. As a result, an unnecessary warning for an oil change can be displayed.
On the other hand, in the second conventional degradation-determining system, in the state in which the ignition switch has been turned off and the engine hood is open, even when the oil level rises for a reason other than replenishment of the oil engine, the level sensor can be switched on. As a result, it is erroneously judged that engine oil has been replenished, and hence the time to change the engine oil is erroneously prolonged. Particularly when the oil level is close to the lower limit level, even a slight change in the oil level turns off the switch of the sensor, which makes the above problem more serious.
This problem can be solved by accurate detection of the oil level. However, to detect the oil level accurately, it is necessary to use an oil level sensor for detecting the oil level linearly, and further in combination with the oil level sensor, a vehicle inclination sensor for detecting an inclination of the vehicle, so as to compensate for the influence of an inclination of the vehicle on a result of the detection by the oil level sensor. These sensors are expensive, which results in an increase in manufacturing costs.